This core breeds, screens and maintains the genetic hypercalciuric stone forming (GHS) rats. Using a standard protocol each GHS rat is screened for hypercalciuria and the most severely hypercalciuric rats are used to breed the next generation. Currently we have carried out this breeding and screening for 51 generations (as of January 1999) and every rat from GHS parents is hypercalciuric. The GHS rats excrete approximately 9-10 times as much urinary calcium as controls. This core breeds and supplies GHS rats to the Projects and Cores in this Program Project as outlined in the proposal. As needed, rats are placed on matched high and low calcium and phosphate diets and appropriate urine collections are performed.